Measurement and calibration of AC signals is much more difficult than in the case of DC. One technique is to apply the AC signal to a heater element adjacent to or contacting a temperature sensor such as a bimetallic junction thermocouple, which comprises two wires that are joined at both ends. According to the Seebeck effect, heating one such junction will induce a current in the wires. (In practice, a very high impedance device typically is interposed in the circuit of the two wires so that current flow is negligible). The joule heating in the heater, caused by the AC signal, induces a voltage in the thermocouple. This thermocouple voltage is proportional to the temperature difference between the bimetallic junction adjacent to the heater element and the cold junction temperature.
In one type of known method for measurement of AC voltages, the unknown AC signal and an easily measureable DC signal are separately applied to a heater associated with a electrothermal sensor, such as a thermocouple. First, when the AC signal is connected to the heater, the thermocouple voltage is nulled to a stable voltage source by adjustment of a potentiometer, which is adjusted to apply an equal and opposite voltage to that generated by the thermocouple. Then, without changing the potentiometer setting, an easily measured DC voltage is substituted for the AC signal applied to the heater element, which again induces a voltage in the thermocouple. The DC voltage amplitude is increased until the voltage generated by the thermocouple is again equal and opposite to the voltage previously set by the potentiometer during the AC measuring phase. When this null condition is reached, the amplitude of the DC voltage is measured. This DC amplitude is thereby equal to the RMS amplitude value of the AC signal.
Various methods and apparatus for AC signal measurement or calibration to a thermocouple are disclosed in U.S, Pat. No. 3,723,845 of James J. Duckworth, entitled "True RMS to DC Converters," issued on Mar. 27, 1973 and assigned to the assignee of this application; U.S. Pat. No. 4,659,910 of Arthur J. Harrison, Jr. et al., entitled "AC-DC Transfer Standard Temperature Sensor Reversal Error Compensation Circuit" and copending Ser. No. 580,450, now U.S. Pat. No. 4,695,793, assigned to the assignee of this application.